1. Field of the Invention
The present invention relates to the reduction of iron from the ferric to the ferrous state in solutions of strong acids, such as those used in oil and gas well stimulation and in the acid cleaning of industrial equipment such as boilers, heat exchangers, and the like. More particularly, it relates to this chemical reduction as an aid in minimizing the formation of solid reaction products during such procedures. Most particularly, it relates to the use of this chemical reduction in the minimization of the occurrence of iron-induced acid sludging of certain crude oils in the course of well stimulation.
2. Description of the Relevant Art
It is well established in the prior art that dissolved iron in the ferric oxidation state in aqueous acid solutions can lead to the formation of ferric iron-containing compounds in the bearing solution which produce insoluble iron solids when the pH of the acid solution increases to a value greater than approximately 4. In this regard, U.S. Pat. No. 4,683,954 to Walker and U.S. Pat. No. 5,084,192 to Dill teach that ferric compounds such as ferric hydroxide begin to precipitate from hydrochloric acid solution when the pH of the acid increases to a value of about 2.5 and greater and that precipitation is complete when the solution's pH is about 3.5. This precipitation phenomenon becomes a serious problem when an acid, such as hydrochloric acid, containing dissolved ferric iron is being used to react with a subsurface, acid soluble, calcareous formation, such as limestone, wherein the acid reaction causes the pH of the acid solution to typically spend to a value greater than the 4 and 5 range.
In addition to the precipitation problem discussed above which can be caused by the presence of ferric ion in acid, it is taught by several authorities that hydrochloric acid, particularly when at high concentrations of about 15% and greater, can cause the development of sludge when the acid is placed in contact with certain types of crude oil. The sludge formation problem is exacerbated when the acid which is in contact with the crude oil also contains ferric ion.
For purposes of this invention, sludge is defined as a solid material formed in crude oil containing asphaltenes and maltenes which constituents may, under certain conditions as pointed out above, precipitate from the crude oil. Sludge formed in crude oil while the crude oil is in a formation can render very difficult the task of recovery of the oil from the formation. Crude oil containing quantities of asphaltenes and maltenes subject to the production of sludge is referred to herein as sludging crude.
Accordingly, the sludging problem specifically addressed herein is caused by the combination of acid, especially high concentration hydrochloric acid, and ferric ion in contact with a sludging crude. This problem is particularly severe when the sludge is produced during formation acidizing.
Formation acidizing or simply, acidizing, is a well known method used to increase the flow of fluid from a subterranean formation. According to conventional practices, the underground formation is contacted with an acidic composition to react with and dissolve material contained therein for the purpose of increasing the permeability of the formation. The flow of fluid from the formation is therefore increased because of the increase in formation permeability caused by the dissolution of the material. A known method of acidizing comprises the steps of conducting an acid composition to the formation through tubing disposed in a borehole that penetrates the formation; forcing the acid composition into contact with the formation and permitting the acid to react with and dissolve certain materials contained therein to enlarge passages through the formation and thus increase the permeability of the medium so treated.
It is apparent that the object of formation acidizing, which is to increase formation permeability, can be frustrated if the very acid composition employed in the treatment to achieve that object produces an environment which fosters the development of solid material which can instead fill and plug pore spaces intended to be enlarged in the treated formation. Instead of enhancing fluid production, the consequent result is a failure to increase production, and even possibly decrease formation permeability.
In the context of formation acidizing, ferric ion can be introduced into the acid as a result of reaction between ferric compounds, such as rust and millscale, contacted in such locations as the tanks used to store the acid and/or transport it to a well that requires acidizing. Most common, such ferric compounds may be encountered on the interior walls of the conduit which conducts the acid down to the formation, reaction of the acid with ferrous compounds in the formation followed by oxidation of ferrous ion to ferric ion, and reaction between the acid and formation minerals that include ferric compounds, such as goethite, FeO(OH), magnetite, Fe.sub.3 O.sub.4, and hematite, Fe.sub.2 O.sub.3.
Solutions to the problems of precipitation of ferric iron compounds from spent acid and the formation of sludge induced by the contact between ferric ion and acid with sludging crude revolve about the control of ferric ion in the acid solutions and/or the elimination of those ions from the solution. One suggested mitigating procedure is the removal of ferric compounds from the metal conduits through which the acid solutions are conducted down-hole, such as by a process known as pickling, prior to the conduit's utilization in acidizing procedures.
As long ago as 1939, a portion of this iron problem was recognized by Grebe: his U.S. Pat. No. 2,175,081 teaches of sulfur dioxide or sulfurous acid to lessen the precipitation of iron salts as pH increases after an acid job. Grebe does not address the problem of sludge formation in his patent. The Dill patent ('192) discloses the use of a blend of formic acid and acetic acid, in combination with anti-sludge agents and iron control agents. U.S. Pat. No. 4,823,874 discloses the use of anti-sludging agents such as quaternary ammonium salts of fatty amines in hydrochloric acid. U.S. Pat. No. 4,574,050 to Crowe discloses the use of an iron control agent, such as ascorbic acid and erythorbic acid, in hydrochloric acid. U.S. Pat. No. 5,063,997 to Pachla appears to disclose the reduction of ferric ion to ferrous ion in hydrochloric acid with hypophosphorous acid and catalyst material selected from cupric and cuprous compounds.
In U.S. Pat. No. 5,445,221 to Vinson, the reduction of ferric ion to ferrous ion in hydrochloric acid is disclosed with certain sulfur-containing, non-ionic, organic compounds in combination with a separate catalyst material selected from copper and vanadium compounds. The disclosure of the '221 patent is detailed and accurate with respect to the background of that invention and the needs for ferric ion reduction for applications in oil field settings. For these same purposes of background information regarding the needs and applications for ferric ion and iron reducing capabilities, U.S. Pat. No. 5,445,221 is expressly incorporated herein by reference. U.S. Pat. No. 6,306,799 discloses similar products, and teaches the use of copper and iodine compounds along with organic sulfur compounds for carrying out these same goals.